Mounting an optical device on silicon may be one of the important factors in reducing the size and power consumption of optical transceiver having a large capacity. Since an optical device on silicon is capable of employing an optical waveguide with a large relative refractive index difference, it may be more advantageous to use the optical device on silicon than to use other materials in the reduction in size of an optical transceiver. Further, since the optical device on silicon is easily integrated with an electronic circuit, it may be possible to integrate a large number of optical transceivers into one chip.
In particular, a modulator has, among other optical devices, properties that greatly affect power consumption and size of an optical transceiver. Specifically, a ring modulator itself is small, uses a small modulation voltage, and has a small optical loss, it is advantageous to use the ring modulator for the reduction in size and power consumption of an optical transceiver. However, a wavelength band of the ring modulator may be narrow.
For example, US Patent Publication No. 2009/0169149 (Patent Document 1) discloses a technology to match a wavelength of light input to a ring resonator and a resonant wavelength. In this technology, output of monitor light is detected, and a heater is controlled based on a monitor current. The monitor current is fed back to the heater to adjust a temperature of the ring resonator, and the resonant wavelength is shifted so as to be consistent with the wavelength of the input light.
FIG. 1 is a schematic diagram of a monitor current feedback ring modulator. Modulation signals supplied via a driver circuit to modulation electrodes 1004 and 1005 change a resonant wavelength of a ring resonator 1003. Light input to a waveguide 1001 is output to a waveguide 1002 if the light resonates with the ring resonator 1003. Light input to a waveguide 1001 is supplied as monitor light to a photodetector (PD) if the light does not resonate with the ring resonator 1003. The monitor current output from the photodetector is supplied to a heater 1007 via a feedback circuit. The heater 1007 is controlled such that the monitor current becomes minimum.
When modulation efficiency is attempted to be increased by using the ring modulator, the transmittance close to the resonant wavelength may dramatically change with respect to the wavelength of input light. As a result, slight shift of the wavelength from the resonant wavelength may lead to an area exhibiting no transmittance change. The same applies to the monitor current. When the wavelength of the input light slightly separates from the resonant wavelength, a change in the monitor current due to the wavelength change cannot be detected, which makes it difficult to perform wavelength control using the heater. That is, it may be difficult to simultaneously achieve improvement of the efficiency of the modulation by enhancing the resonance and wide-range wavelengths.